The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then salted with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives at the coating site to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking.
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.
All current commercial cathodic electrocoat processes must be cured at a high temperature (e.g., 325.degree. F. to 360.degree. F.). However, at these high bake temperatures it is not possible to coat many plastic substrates because they tend to distort at high temperature. Nevertheless, there is a push within the automotive industry to go toward plastic substrates for certain uses. Thus there is a need for an electrocoat process that will allow curing at lower temperatures (e.g., 250.degree. F.-275.degree. F.) so that the baking process will not distort the plastic substrate.
The push toward low cure electrocoat systems has thus for been frustrated by bath instability, film roughness, poor coating corrosion resistance and poor chip resistance. These characteristics are interrelated and are thought to be at least partially caused by premature curing of the film while still in the electrocoat bath.
It is very important that the electrodeposited layer be of high quality even though it typically will be covered with top coats. Defects in the electrodeposited layer such as cratering or roughness may be evident through the top coats.
In order that plastic substrates may be used in electrocoat processes there is a need for an electrocoat system that will allow for low cure but also have a stable bath, smooth film, good corrosion resistance and acceptable chipping characteristics.